Zinc-coated steel cord with improved fatigue resistance

ABSTRACT

A steel cord for the reinforcement of thermoplastic elastomers, the steel cord including more than one strand, each strand including two or more steel filaments, at least some of said the steel filaments being provided with a zinc coating. The zinc coating has a thickness lower than two micrometer and an alloy layer zinc-steel is present between the zinc coating and the steel.

FIELD OF THE INVENTION

The present invention relates to a steel cord adapted for thereinforcement of thermoplastic elastomers and to a composite. Thecomposite comprises a thermoplastic elastomer such as polyurethane asmatrix and the steel cords as reinforcing material. The steel cord is amulti-strand steel cord, i.e. a steel cord comprising more than onestrand.

BACKGROUND OF THE INVENTION

Steel cords are widely known to reinforce rubber products such as tiresand conveyor belts. To a lesser extent, steel cords are also known toreinforce thermoplastic elastomers such as polyurethanes. The adhesionmechanism between the steel cords and the rubber products issubstantially different from the adhesion mechanism between steel cordsand thermoplastic elastomers. The adhesion between steel cords andrubber is mainly a chemical adhesion based on the bonds created duringvulcanization between a conventional copper alloy coating on the steelcord and the rubber. Although chemical adhesion is not excluded betweensteel cords and thermoplastic elastomers, the adhesion mechanism issubstantially based on a mechanical anchoring between the steel cordsand the matrix.

Due to this basic difference, steel cords for rubber reinforcement haveknown another evolution than steel cords for reinforcement ofthermoplastic elastomers.

With respect to rubber reinforcement, there has been an evolutiontowards single-strand cords such as compact cords, lesser filaments inone steel cord (even ending up in single filament reinforcements),thicker filament diameters.

With respect to reinforcement of thermoplastic elastomers, themulti-strand steel cords, i.e. steel cords comprising more than onestrand, have remained the standard because the rougher outer surface ofsuch a multi-strand steel cord offers more mechanical anchoring in thematrix than a single-strand steel cord.

Another consequence of the different adhesion mechanism between steelcords adapted for the reinforcement of rubber products and steel cordsadapted for the reinforcement of thermoplastic elastomers, is the typeof coating applied to the steel filaments. Whereas steel cords forrubber reinforcement, particularly for tires, have a conventional copperalloy coating such as brass, steel cords for the reinforcement ofthermoplastic elastomers conveniently have a zinc or zinc alloy coating.Such a zinc coating, however, has its drawbacks.

A first drawback is that a suitable level of corrosion resistance isdifficult to combine with an acceptable level of fatigue resistance.Indeed increasing the thickness of the zinc coating leads to an increaseof the corrosion resistance, which is an advantage, and to a decrease infatigue resistance, which is a disadvantage, and vice versa.

A second drawback is that a zinc coating creates a lot of zinc dust andzinc particles during the downstream working of the zinc coatedfilaments such as the cold drawing and the twisting into the strands andcord.

SUMMARY OF THE INVENTION

It is an object of the invention to avoid the drawbacks of the priorart. It is another object of the invention to provide a steel cord witha coating, which has both an acceptable degree of fatigue resistance andan acceptable degree of corrosion resistance.

According to the invention, there is provided a steel cord adapted forthe reinforcement of thermoplastic elastomers. The steel cord is aso-called multi-strand steel cord, i.e. the steel cord comprises morethan one strand. Each strand comprises two or more steel filaments. Atleast some of the steel filaments are provided with a zinc coating. Thezinc coating has a thickness lower than two micrometer, preferably lowerthan one micrometer, e.g. 0.5 μm. An alloy layer zinc-steel is presentbetween the zinc coating and the steel.

The typical zinc coating, i.e. a relatively thin zinc coating (commonzinc coatings have a thickness greater than 3 to 5 micrometer) incombination with the presence of a transition alloy layer zinc-steel hasfollowing advantages.

The thin coating has the advantage of producing less dust of zinc duringthe downstream drawing and twisting of the steel cord. The decrease inzinc dust and zinc particles at the surface of the steel cord leads to abetter mechanical anchoring in the thermoplastic elastomer.

The presence of the transition layer of a zinc-steel alloy between thesteel and the zinc increases the corrosion resistance of the steelfilaments and increases the adhesion between the zinc coating and thesteel. The presence of this alloy layer leads to an even furtherreduction of zinc dust and, as a consequence, to a better anchoring ofthe steel cord with the zinc coating in the thermoplastic elastomer.

With respect to the cord core, two alternatives are possible: a cordcore in the form of a core strand with two or more filaments and a cordcore in the form of a plastic material, e.g. of the same type and natureof the thermoplastic elastomer of the matrix.

Amongst the various constructions tested by the inventors, a 7×7construction with following parameters has proved to provide excellentresults with respect to mechanical anchoring, fatigue resistance,fretting behavior and corrosion resistance:d ₁+6×d ₂+6×(d ₂+6×d ₃)with

-   -   d₁>1.05×d₂    -   d₂>1.05×d₃

The invention also relates to a composite reinforced by a steel cord asdescribed hereabove. An example of such a composite is a belt, e.g. agrooved belt, with polyurethane as matrix

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described into more detail with reference tothe accompanying drawings wherein

FIG. 1 shows a schematic view of a composite belt according to theinvention;

FIG. 2, FIG. 3 and FIG. 4 all show cross-sections of steel cords for thereinforcement of a composite according to the invention;

FIG. 5 shows a cross-section of a steel filament with a zinc coating.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows a schematic view of a composite belt 10 according to theinvention. The belt 10 is provided with grooves 12, which is a result ofits manufacturing process: the grooves are located at the places whereteeth were guiding the belt 10.

As an alternative, grooves or teeth may also be present in thelongitudinal direction of the belt so as to constitute a so-calledV-grooved belt (not shown).

The belt 10 has a matrix material 14 out of polyurethane. Other suitablethermoplastic elastomers are thermoplastic polyolefin homopolymers orcopolymers, olefinic rubbers, block-copolymers of styrene/conjugateddiene/styrene and/or its fully or partially hydrogenated derivative,optionally compounded with a thermoplastic polyolefin homopolymer orcopolymer, or blends of the foregoing. Such thermoplastic elastomers aredescribed in more detail in WO-A-99/55793 (Advanced Elastomer Systemsand N.V. Bekaert S.A.).

The belt 10 is further reinforced with several steel cords 16, which arelying in parallel adjacent to each other.

Steel cords adapted for the reinforcement of thermoplastic elastomershave, either alone or in combination, following features:

-   -   steel filaments with a diameter ranging from 0.05 mm to 0.80 mm,        preferably from 0.06 mm to 0.40 mm;    -   the steel filaments have a steel composition which is along the        following lines: a carbon content ranging from 0.60% to 1.05%, a        manganese content ranging from 0.10% to 1.10%, a silicon content        ranging from 0.10% to 0.90%, sulfur and phosphorous contents        being limited to 0.15%, preferably to 0.10%;    -   additional micro-alloying elements such as chromium (up to        0.20%-0.40%), copper (up to 0.20%) and vanadium (up to 0.30%)        may be added;    -   although not strictly necessary since the mechanical anchoring        is the main adhesion mechanism, the steel cords may be provided        with an additional chemical adhesion system such as described in        the already mentioned WO-A-99/55793 or such as provided in        WO-A-99/20682 (N.V. Bekaert S.A.).

Starting from a steel wire rod with the above-mentioned compositionmakes a composite product according to the invention. The steel rod iscold drawn to the desired filament diameters. The subsequent colddrawing steps may be alternated by suitable thermal treatments such aspatenting, in order to allow for further drawing. Once the finaldiameters are obtained, the drawn filaments are twisted to a strand, anda number of strands are twisted to a steel cord. Conventional apparatussuch as double-twisters (“bunching apparatus) or such as tubular rotarymachines (“cabling apparatus) may do the twisting operations. A multipleof the twisted steel cords are then drawn and straightened from supplyspools, laid in parallel adjacent each other and fed through insertholes to an extrusion apparatus where the thermoplastic elastomer isadded.

Other composite products according to the invention with a parallel andstraight pattern of steel cord reinforcement are: sheet-linings, snap-onprofiles, cut-resistant flexible and protective strips, handrails etc.

FIG. 2 shows the cross-section of a steel cord 16, which has proved tobe a preferable reinforcement for the composite 10.

Steel cord 16 comprises a core strand 18 and six outer strands 20. Corestrand 18 comprises a filament 22 as core and six filaments 24 twistedaround the filament 22. The diameter d₁ of filament 22 is at least 5%greater than the diameter d₂ of the filaments 24. The outer strands 20each comprise a filament 26 as core and six filaments 28 twisted aroundthe filament 26. The filament 26 may have the same diameter d₂ as thefilaments 24. The diameter d₂ of filament 26 is at least 5% greater thanthe diameter d₃ of filaments 28.

An example of a preferable construction is (all diameters beingexpressed in mm):(0.21+6×0.19)+6×(0.19+6×0.175)  (a)

The steel cord filaments are all provided with a thin zinc coating withan average thickness of about 0.5 μm to 1.0 μm.

The table hereunder summarizes the mechanical properties of this cord.

TABLE 1 Twist Twist direction Z direction S Linear density (g/m) 10.1310.03 Breaking Load F_(m) (N) 3139 3139 Tensile strength R_(m) (MPa)2436 2459 Yield strength at 0.2% permanent 2067 2103 elongation R_(p0.2)(MPa) R_(p0.2)/R_(m) (%) 85 86 Modulus of elasticity E_(mod) (MPa)139103 145156 Permanent elongation at maxim load 1.14 1.25 Ag (%)Percent total elongation at fracture A_(t) 2.95 2.95 (%) Preformationdegree of outer strands 97 98 (%)

In a belt endurance test with following test conditions:

-   -   pulley diameter r=40 mm    -   axial load=150 kg    -   frequency=100 cycles per minute

This cord (a) has been compared with following cord:(0.20+6×0.175)+6×(0.175+6×0.175)  (b)

For cord (b) friction corrosion was visible after 2 150 000 cycles,whereas for cord (a) no friction corrosion was visible after 2 150 000cycles.

The above steel cord corresponds to the general formula: 7×7. Othersuitable steel cord constructions are (subject to the required opennessand deformation degree of the outer strands):

-   -   19+8×7    -   7×19    -   1×7+7×7    -   19+9×7    -   1×3+5×7

FIG. 3 illustrates an alternative steel cord 16 for the reinforcement ofa composite according to the invention. The main difference with steelcord (a) of FIG. 1 is that the core of the core strand is now a plasticmaterial 30, e.g. a polyurethane of the same family as the matrixmaterial to be reinforced. This plastic material 30 is necessary toprevent the filaments 24 from forming a completely closed structure. Thesteel filaments are also all provided with a thin zinc coating with anaverage thickness of about 0.5 μm to 1.0 μm.

FIG. 4 illustrates yet another alternative steel cord 16 for thereinforcement of a composite according to the invention. The core of thesteel cord 16 is now completely formed by a plastic material 32, whichis thick enough to prevent the strands 20 from forming a closedstructure and from contacting each other. So plastic material 32 helpsto obtain an open structure and helps to avoid fretting between adjacentstrands 20.

On a strand level, the core of each strand 20 is formed by anotherplastic material 34. Seven outer filaments 28 surround this otherplastic material 34.

Here again, the steel cord filaments are all provided with a thin zinccoating with an average thickness of about 0.5 μm to 1.0 μm.

FIG. 5 shows the cross-section of a zinc coated steel filament 36 inaccordance with the invention. Steel filament 36 has a steel core and azinc coating 40 of less than 2 micrometer. Between the steel core 38(about 100% steel) and the zinc coating 40 (about 100% zinc), atransition layer 42 of a zinc-steel alloy is present.

Such a transition layer can be obtained if, in contrast with anelectrolytic deposition method of zinc, the steel wire is zinc coated bymeans of a hot dip operation. In a hot dip operation the steel wiretravels through a bath of molten zinc and leaves the bath zinc coated.

If the steel wire leaves the bath vertically, a thick zinc layer ispresent together with a relatively thick and rough zinc-steel alloylayer. Both the thick zinc layer and the steel-zinc alloy layer havedisadvantages. The thick zinc layer produces too much zinc dust and zincparticles during the subsequent drawing and twisting steps. Thesteel-zinc alloy layer, although increasing the corrosion resistance, isquite brittle and results in a decrease in fatigue resistance.

If the steel wire leaves the bath under a small angle with respect to ahorizontal line and if the leaving steel wire is wiped mechanically,e.g. by means of a ceramic cloth, three differences are noticed withrespect to the vertical leaving:

-   (a) a reduced thickness of the zinc coating;-   (b) a reduced thickness of the zinc-steel transition layer;-   (c) a reduction in the roughness of the zinc-steel transition layer.

All three differences together lead to a reconciliation between the twoconflicting requirements of sufficient fatigue resistance and sufficientcorrosion resistance.

The reduced thickness of the zinc coating (a) and the reduced thicknessof the zinc-steel transition layer (b) leads to an increase in thefatigue resistance.

The presence of the zinc-steel transition layer (b) results in anacceptable corrosion resistance, which is significantly higher than incase there is no zinc-steel transition layer.

The reduced thickness of the zinc coating (a) and the reduction in theroughness of the zinc-steel transition layer (c) leads to lessproduction of zinc dust and zinc particles during the subsequent colddrawings steps. The reduction in thickness of the zinc coating (a), thecold drawing does not lead to more fractures during the cold drawingsteps and twisting steps since the brittle transition layer is alsoreduced in thickness (b) and since the transition layer has a reducedroughness (c).

Table 2 hereunder gives and indication of the difference in fatigueresistance between a reference steel cord {circle around (1)} composedof steel filaments hot dip coated with zinc and leaving vertically thezinc bath and an invention steel cord {circle around (2)} hot dip coatedwith zinc and leaving the zinc bath under a small angle with thehorizontal plane.

TABLE 2 Reference Invention steel cord {circle around (1)} steel cord{circle around (2)} Weight of zinc (g/kg) 80.1 26.1 Thickness of zinc(μm) 3.3 1.1 Dry Hunter fatigue resistance 100 131 (relative terms) WetHunter fatigue resistance 100 117 (relative terms) Number of bendingcycles before 100 472 fracture of embedded cord under tensile load(relative terms)

1. A steel cord adapted for the reinforcement of thermoplasticelastomers, said steel cord comprising more than one strand, the strandscomprising two or more steel filaments, at least some of said steelfilaments being provided with a zinc coating, wherein said zinc coatinghas a thickness lower than two micrometers and a zinc-steel alloy layeris present between the zinc coating and the steel.
 2. A steel cordaccording to claim 1 wherein said zinc coating has a thickness lowerthan one micrometer.
 3. A steel cord according to claim 1, wherein saidsteel cord comprises a cord core and three or more outer strands twistedaround said cord core and contacting said cord core.
 4. A steel cordaccording to claim 3 wherein said cord core is a plastic material.
 5. Asteel cord according to claim 3 wherein said cord core is a core strandwith two or more filaments.
 6. A steel cord according to claim 5,wherein said steel cord is a steel cord with a core strand and six outerstrands according to the formula:d ₁+6×d ₂+6×(d ₂+6×d ₃) with d₁>1.05×d₂ and d₂>1.05×d₃.
 7. A compositecomprising a thermoplastic elastomer as matrix material and a steel cordaccording to claim 1 as reinforcing material.
 8. A composite accordingto claim 7 wherein said composite is a belt.
 9. A composite according toclaim 8 wherein said belt is a grooved belt.
 10. A composite accordingto claim 7, wherein said thermoplastic elastomer is a polyurethane. 11.A method of manufacturing a steel cord according to claim 1, comprisingthe following actions: (a) obtaining a steel filament; (b) passing thesteel filament through a bath of molten zinc such that the steelfilament leaves the bath under a small angle with respect to ahorizontal line and is coated with zinc, and mechanically wiping thesteel filament coated with zinc, wherein, after wiping, the steelfilament has a zinc-steel alloy layer covered by a zinc coating having athickness lower than one micrometer; and (c) forming a steel cordaccording to claim 1 with steel filaments coated with zinc obtained byactions (a)-(b).
 12. A method of manufacturing a steel cord according toclaim 2, comprising the following actions: (a) obtaining a steelfilament; (b) passing the steel filament through a bath of molten zincsuch that the steel filament leaves the bath under a small angle withrespect to a horizontal line and is coated with zinc, and mechanicallywiping the steel filament coated with zinc, wherein, after wiping, thesteel filament has a zinc-steel alloy layer covered by a zinc coatinghaving a thickness lower than one micrometer; and (c) forming a steelcord according to claim 2 with steel filaments coated with zinc obtainedby actions (a)-(b).
 13. A method of manufacturing a composite accordingto claim 7, comprising the following actions: (a) obtaining a steelfilament; (b) passing the steel filament through a bath of molten zincsuch that the steel filament leaves the bath under a small angle withrespect to a horizontal line and is coated with zinc, and mechanicallywiping the steel filament coated with zinc, wherein, after wiping, thesteel filament has a zinc-steel alloy layer covered by a zinc coatinghaving a thickness lower than one micrometer; and (d) forming thecomposite according to claim 7 with steel filaments coated with zincobtained by actions (a)-(b).
 14. A method of manufacturing a compositeaccording to claim 8, comprising the following actions: (a) obtaining asteel filament; (b) passing the steel filament through a bath of moltenzinc such that the steel filament leaves the bath under a small anglewith respect to a horizontal line and is coated with zinc, andmechanically wiping the steel filament coated with zinc, wherein, afterwiping, the steel filament has a zinc-steel alloy layer covered by azinc coating having a thickness lower than one micrometer; and (d)forming the composite according to claim 8 with steel filaments coatedwith zinc obtained by actions (a)-(b).